Good questions, Dingo. Unfortunately, it's true that brass-plated steel brushes often masquerade as brass brushes - you can't trust certain manufacturing super-powers. Beware high spindle speeds and wire brushes. It's recommended to run new brushes at full speed for a couple of minutes to encourage the loose 'bristles' to fly out before using in earnest. Wear eye protection! Copper and its alloys (including brass) in the eye is a medical emergency. Copper destroys eyes! If there's ANY chance you've suffered a penetrating eye injury, get to hospital double-quick!
Whilst brass brushes seem attractive for their softness, compared to steel reed material, the possibility of brass contamination of the steel surface is real. This is theoretically a potential problem. Whether this is actually likely to encourage dissimilar metal corrosion in practice, I don't know. Best to avoid the possibility of moisture, of course.
I wouldn't worry about using stainless steel brushes (gently) in a Dremel, (although see next para). They're easy to get, and relatively cheap. I've used them a lot. Reed material is pretty hard, so the risk of damage is small. I'd think that if there were a need to be so rough with the reeds that damage was a risk, they'd probably be too far gone. (Actually, I'm making an assumption here. I'm assuming that reeds will be spring steel. I'm aware that other materials are used for musical reeds, but would think that, if it looks like steel, it will have a moderately hard surface. If I'm wrong about this, someone will punish me for my ignorance, I expect!)
Glass-fibre 'pencils' are great for this type of fine work. No risk of a rapidly-moving 'bristle' snagging in the gap between reed and reed plate.